My suggestion is similar to another comment: educate yourself. If you are his grandparent and find you are too "old" to learn the topic (which I would consider a cop-out, though it's your choice to decide), your fall back is the home-schooling community.
It seems as if his parents are somewhat neglectful since they haven't taken the time to address his reading level (as others, I am assuming no disability), so get other people to help you. It seems everywhere I go there is a hidden home-school community; it might be hard to find, but it is likely there. If you are having trouble finding the community, I'd suggest asking a local high-school educator for introductions to parents (s)he happens to know that home school. I think you will find that other parents are very kind and helpful. Often I see people "trading" their children to other parents who happen to know more on a subject than them. If you are expected to participate in this by teaching students in return for chemistry lessons, don't downplay your own education.
If your a grandmother you are likely at _least_ over 50 years old. You have a lot of life experience you can share. Teaching, especially home-schooling, is not necessarily about science, math, and geography. It is also about developing young well rounded minds. Teach them something you know, if that's just knitting, then by god teach them knitting. You will teach them patience, spacial co-ordination, and social aspects (people go to knitting clubs to talk, not just knit). Above all, though, educate yourself. Nothing helps you understand material better than teaching it. Seeing other peoples views on things that you could have missed. This is not just an opportunity for your grandson to have an excellent education in things he enjoys, but it is an opportunity for you to continue you your amassed world knowledge with further specifics.
You are never too old to learn something new.